Optical disks known in the art include rewritable optical disks of the magneto-optical, phase change, and other modes as well as write-once type optical recording disks. Most optical recording disks have a recording layer formed on a substrate, and are generally recorded and reproduced by directing a laser beam or similar light through the substrate. Therefore, the substrates used are formed of transparent materials, for example, glass and resins. For a weight reduction and ease of formation of tracking grooves and pits, conventional optical recording disks use resinous substrates with polymethyl methacrylate and polycarbonate being frequently employed.
Among them, polymethyl methacrylate has excellent optical properties including a low coefficient of optical elasticity and a very low birefringence, but is poor in dimensional stability due to high water absorption and low heat resistance.
Therefore, polycarbonates having low water absorption and relatively high heat resistance are used most often. However, the polycarbonates have a high coefficient of optical elasticity and often a high birefringence since their polymer chain tends to orient in a flow direction. For achieving a higher recording density and higher transfer rate, it was attempted in recent years to use disks of a larger diameter of at least 200 mm and carry out recording/reproducing operation at a high number of revolutions of at least 1,800 rpm. In such large diameter substrates, birefringence is incurred during injection molding of polycarbonate which is further increased through application of stresses during high speed rotation, resulting in a lowering of reproduced outputs. The polycarbonate is not fully low in water absorption so that large size substrates will undergo substantial distortion. Some lack of rigidity causes axial runouts and focus tracking errors during high rotational speed driving at 1,800 rpm or higher, especially at 3,000 rpm or higher. In addition, the low solvent resistance of polycarbonate is a problem when recording layers of the coating type are applied.
To meet the recent demand for high rotational speed driving at 1,800 rpm or higher, especially at 3,000 rpm or higher, substrates having higher mechanical strength should be used. Glass substrates have the high risk of failure when the disks are accidentally dropped or hit. Particularly when chemically tempered glass of high strength is used, it can fracture into scattering fragments. Glass substrates also have the problem that grooves useful as a pre-format information carrier cannot be formed.
To protect the recording layer from mechanical damage as by scratching, it is convenient to provide a protective plate on the recording layer or to mate a pair of substrates such that the recording layer is inside.